A power can be calculated from a voltage on both ends of a load and a current carried to the load. Specifically, the power can be obtained by multiplying a load current carried to the load by a load voltage applied to the load. However, a power using an alternating-current (AC) power supply cannot be simply calculated since the power includes an effective power that is consumed as an energy and an ineffective power that is not consumed as the energy.
Conventionally, there is generally known the following method for detecting a power. A current detection transformer (CT) calculates the load current, and a voltage detection transformer (VT) calculates the load voltage. In addition, an electronic circuit or a microcomputer multiplies the current by the voltage based on the above-stated principle. With this conventional method, an AC power can be measured but a direct-current (DC) power cannot be measured because of characteristics of the CT and the VT.
Meanwhile, there is known the following method capable of measuring both the DC power and the AC power and directly detecting the power.
The method is executed by a power meter. The power meter is an indicator power meter using an indicator and referred to as an electrodynamometer type power meter. The indicator is moved using a torque acting on between a current coil and a voltage coil, a graduation on the meter indicated by the indicator is read, thereby measuring the power. This power meter measures the power by allowing a user to visually recognize the graduation. Due to this, it is difficult to incorporate this power meter into a recently required automated apparatus and to use it sequentially with a digital signal processing. Actually, the power meter cannot be used for these purposes.
Furthermore, there is known the following power sensor technique (see, for example, Patent Literature 1). With this power sensor technique, a plane of polarization of a Faraday effect optical element is rotated by a magnetic field of the load current, and a light proportional to the load voltage is transmitted by the Faraday effect optical element. A light correlated to both the load current and the load voltage is obtained. The light is subjected to an optical-electric conversion, thereby obtaining an electric signal correlated to the power. This conventional technique has, however, the following disadvantages. This conventional technique cannot ensure high accuracy since errors are accumulated by an electric-optical conversion and the optical-electric conversion. In addition, since this conventional technique uses an optical system, it is not only necessary to employ an expensive optical element but also necessary to use labor and time for adjustment of the expensive optical element. As a result, a cost is too high for the accuracy obtained.
There is further known the following power meter technique (see, for example, Patent Literature 2). With this conventional technique, a current proportional to the load voltage is carried to an input terminal of a Hall element, and a magnetic flux generated by the load current is applied to the Hall element. A voltage proportional to both the load current and the load voltage is thereby obtained from an output of the Hall element. This conventional technique has, however, the following disadvantages. Because of use of the Hall element, sensitivity to the load current is low and the sensitivity has a large fluctuation relative to temperature. Besides, since an irregularity is present among Hall elements, the power can be obtained only at low sensitivity with low accuracy.
Moreover, there is known the following power meter technique (see, for example, patent Literature 3). With this conventional technique, a magnetic field generated by a current proportional to the load voltage and a magnetic field generated by the load current are applied to the same core, and two sensors which detect respective magnetic fluxes by magnetic sensors are provided. One of the sensors calculates a difference between a signal corresponding to the load voltage and a signal corresponding to the load current. The other sensor calculates a sum therebetween. Further, there is a technique that an electronic circuit is used to perform a square difference calculation on results of the both sensors, thereby measuring the power (see, for example, Patent Literature 3). This conventional technique has, however, the following disadvantages. It is necessary to employ as many transformers as those employed in the method for detecting the power using the CT and the PT because two transformers in which a core and a coil are in combination must be employed. With this conventional technique, flux gate type means using the magnetic sensors is adopted as means for detecting the magnetic fluxes of the transformers. Since the two transformers are employed, it is necessary to provide two flux gate circuits as well as a computing circuit and the like. As a result, a circuit scale is inevitably made large.
Patent Literature 1: Japanese Patent Application Laid-Open No. 1-162165
Patent Literature 2: Japanese Patent Application Laid-Open No. 11-108971
Patent Literature 3: Japanese Patent Application Laid-Open No. 8-304481